There are a number of different reasons why pacemakers are implanted. Pacemakers may monitor and function only to back up the normal sinus rhythm of the heart. Alternatively, a pacemaker may provide complete control of the pacing of the heart. Rate modulated pacemakers have been developed to help patients adapt to physiological stress by increasing their heart rate. Rate modulated pacing is particularly advantageous for patients with chronic atrial defibrillation or sick sinus syndrome which prevents normal physiological sinus node response to exercise or stressful activities.
Rate modulated pacing is accomplished by providing a sensor capability, either built into the pacemaker or attached as an additional lead, in combination with a pacing rate control algorithm in the pacemaker control memory. One type of sensor presently in use is a motion detector built into the pacemaker. There are currently two types of motion detectors mounted within the pacemaker. The first is a piezoelectric material which is formed in a flat sheet and placed inside the pacemaker housing. Changes in body movement and muscle motion cause deformation of the piezoelectric crystal which produces an output signal. The output signal is dependent upon the amount of motion and can be used as a direct input into the pacing rate control algorithm.
The presently available motion detector devices are capable of providing activity responsive output signals. However, the elimination of false signals, which may result from externally imposed accelerations and motion as compared to the physical activity of the patient, remains a serious problem. For this reason, lead mounted sensors to indirectly measure physical activity have been designed which include sensors affixed to leads extending into the atrium or ventricle, which monitor temperature, blood perfusion, or respiratory rate. However, use of these types of lead-mounted sensors for the rate modulated pacemakers result in the necessity of having an additional implanted lead, and the associated problems and limitations thereof.
Accordingly, it would be beneficial to have an improved accelerometer device mounted within the pacemaker housing to provide an accurate signal indicative of physiological induced stress based on acceleration and motion of the individual, while eliminating or reducing externally imposed false signals.